High power beam tube having depressed potential collector containing field-shaping probe

ABSTRACT

An efficient high power beam tube includes a gridded convergent flow electron gun for forming and projecting a beam of electrons through a centrally apertured accelerating anode to a fly-trap type depressed beam collector structure. A tapered conductive probe is axially disposed within the beam collector for cooperation with a collector focus electrode structure to shape the equipotentials at the mouth of the fly-trap beam collector, in the presence of space charge depression within the collector, such as to retain laminar electron flow and uniform current density transversely across the beam as the beam decelerates and expands into the beam collector structure, whereby increased depressed collector efficiency is obtained.

United States Patent 1 1 11 1 3,780,336 Giebelcr 1451 Dec. 18, 1973 [54] .HIGH POWER BEAM TUBE HAVING 3,153,743 lO/l964 Meyerer 315/5311 DEPRESSED POTENTIAL COLLECTOR CONTAINING FIELD-SHAPING PROBE Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. [75] Inventor. 2:13:11 Henry Glebeler, Sunnyvale, Aggy-"5y: Fal s Lg; C

[73] Assignee: Varian Associates, Palo Alto, Calif. ABSTRACT [22] Filed: 1972 An efficient high power beam tube includes a gridded [21 Appl. No.: 283,431 convergent flow electron gun for forming and projecting a beam of electrons through a centrally apertured accelerating anode to a fly-trap type depressed beam [5217 Us? Cl g? collector structure. A tapered conductive probe is a ti- [511 I t Cl 23/02 ally disposed within the beam collector for coopera- [58 i 3 g 5 tion with a collector focus electrode structure to shape earc b the equipotentials at the mouth of the fly-trap beam collector, in the presence of space charge depression 56 1 Ref 8 C1 d within the collector, such as to retain laminar electron er nces I flow and uniform current density transversely across UNITED STATES PATENTS the beam as the beam decelerates and expands into ,482 7/1969 P i st 315/538 X the beam collector structure, whereby increased de- 2,958,804 ll/l960 Badger 6t 3.] 3l5/5.38 X pressed collector efficiency is obtained 3,644,778 2/1972 Mihran et al. 315/5318 3,368,102 2/1968 Saharian BIS/5.38 X 5 Claims, 2 Drawing Figures l I D\* I ll [1| l I :L PULSED CONTROL 9 28 GRID SUPPLY A :1 2 f 23 r 6 1 l 1 r Q HIGH POWER BEAM TUBE HAVING DEPRESSED POTENTIAL COLLECTOR CONTAINING FIELD-SHAPING PROBE GOVERNMENT CONTRACT The invention herein described was made in the course of or under a contract or subcontract for the U. S. Department of the Air Force.

DESCRIPTION OF THE PRIOR ART Heretofore, high power beam tubes employing a flytrap depressed collector have been employed to obtain increased switch tube efficiency. One such prior art tube is that as disclosed and claimed in US. Pat. No. 3,453,482 to D. H. Preist, issued July I, I969 and assigned to the same assignee as the present invention.

One of the problems with the prior art depressed collector switch tube is that at high beam power a substantial amount of space charge depression occurs within the fly-trap collector. This space charge depression within the collector alters the equipotentials within and at the mouth of the collector, causing reflection of beam electrons back to the accelerating anode which is operating at full anode potential. The collection of these electrons at full anode potential results in a substantial reduction in the efficiency of the tube.

Accordingly, it is desirable to provide an improved depressed fly-trap collector configuration which results in less reflection of primary electrons when the collector is operated at nearly 100 percent depressed voltage.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved high power beam tube having a depressed beam collector.

In one feature of the present invention, the fly-trap depressed beam collector is provided with a centrally disposed tapered electrically conductive probe for co operation with the collector focusing electrode structure at the mouth of the fly-trap collector for shaping the equipotentials at the mouth of the fly-trap collector, in the presence of a substantial space charged depression within the collector, such that at the mouth of the collector the decelerating beam maintains laminar electron flow of generally uniform transverse current density, whereby the efficiency of the tube is substantially increased.

In another feature of the present invention, the tapered collector probe includes a free end tip portion disposed substantially at the mouth of the fly-trap collector, the tip of the probe having a cross-sectional area of less than 1 percent of the cross-sectional area of the mouth of the collector, such that less than 1 percent of the beam is intercepted on the tip of the collector probe.

In another feature of the present invention, the collector probe includes a conical section having a half cone angle of less than 20, whereby secondary electrons emanating from the probe have trajectories which favor collection of the secondaries within the depressible beam collector cavity.

In another feature of the present invention, the collector probe structure is made hollow to accommodate flow ofa fluid coolant therethrough to facilitate cooling of the probe in use.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view. partly in elevation, of an electron beam tube incorporating features of the present invention, and

FIG. 2 is an enlarged plot of equipotentials for that portion of the structure of FIG. 1 delineated by line 22 and depicting the electron trajectories for a decelerating beam of amps at 95 percent collector de pression for a beam having a beam voltage of kV.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown the beam tube 1 of the present invention. The tube includes a Pierce type gun assembly 2 at one end of the tube for projecting a beam of electrons 3 through an accelerating anode electrode 4 to a fly-trap beam collector 5 at the terminal end of the beam path.

The electron gun 2 includes a cathode emitter 6 having an inwardly dished emissive surface 7 facing the ac celerating electrode 4. The emitter surface 7 is preferably spherically shaped and constitutes a section of a sphere of relatively large radius. The emitting surface 7 is preferably dimpled to form a multiplicity of individual concave lesser cathode emitters in the manner as disclosed and claimed in US. Pat. No. 3,558,967 to A. V. Miram, issued Jan. 26, I97] and assigned to the same assignee as the present invention. A cylindrical beam focus electrode 8 coaxially surrounds the outer periphery of the cathode emitter 6 and projects toward the accelerating electrode 4 for focusing the beam through the accelerating electrode 4.

A shadow grid 9 is disposed overlaying the emitting surface 7 of thermionic cathode emitter 6. The centers of theapertures in the shadow grid 9 are aligned with the centers of the spherically concave lesser cathode emitting surfaces (dimples). The shadow grid 9 is operated at essentially the same potential as the thermionic cathode emitter 6 for suppression of emission from the cathode in the undimpled region of the cathode shadowed by the web of the grid 9.

A spherically concave control grid 11 is disposed overlaying the shadow grid 9 with the apertures of the control grid being aligned in registration with the apertures in the shadow grid 9. The control grid 11 is spaced from the cathode emitting; surface 7 by a relatively short distance, as of 0.039 inch, to define a multiplicity of Pierce guns when a positive potential, relative to the cathode, is applied to the control grid 11. The control grid 11 is supported in electrically insulative relation relative to the thermionic cathode 6, as by an in sulator mounted to the surrounding. focus electrode 8. The focus electrode 8 is preferably operated at cathode potential.

The control grid 11, which faces the accelerating electrode 4, is pulsed with a potential which is a small fraction, as of l/30th to I/50th, of the accelerating potential for controlling the flow of electrons from the emitter 6 through the accelerating electrode 4.

The accelerating electrode, as of copper, includes a central beam passageway 14 of circular cross-section and of smaller diameter than the cathode. The beam passageway 14 is preferably longer than its diameter to provide an effective electric shieldl between the control grid 11 and the collector electrode 5. This shielding is beneficial in preventing transient voltages from being coupled from the collector electrode to the control grid ll during transient switching periods of the tube, which occur when it is operated as a switch tube. When the tube 1 is operated as a radio frequency amplifier passageway 14 prevents coupling of RF voltages from the output circuit to the control grid circuit to thereby precluding possible would otherwise lead to instabili ties and oscillation of the tube.

A cylindrical high voltage insulator assembly 15 holds off the high voltage applied between cathode 6 and accelerating electrode 4 and, in addition, permits certain of the various independent electron gun potentials to be applied to various electrodes thereof. A similar cylindrical high voltage insulator 16 is sealed between the aecelerating electrode 4 and the depressible beam collector structure 5 to form a portion of the tubes vacuum envelope and to permit the collector 5 to be operated at a depressed potential nearly equal to the cathode emitter potential, whereby the forward conduction potential drop of the tube 1 is but a small percentage, as of l to percent, of the accelerating electrode potential.

The beam collector structure 5 includes a beam col lecting cavity portion 17 for collecting the beam on the interior surfaces thereof and a centrally apertured end wall 18 defining a beam entrance passageway 19 (mouth) which is of constricted cross-sectional area compared to the collector portion 17 to prevent escape of secondary electrons from the collector back toward the accelerating electrode 4. An annular conductive beam focus nose member 21 projects from the collector wall 18 toward the accelerating electrode 4 for properly shaping the beam decelerating electric field equipotential surfaces at the beam entrance passageway 19.

An conically shaped electrically conductive collector probe member 20 is disposed on the axis of the collector cavity 17. The collector probe is preferably conical and projects from the remote endwall of the collector 5 toward the mouth 19 of the fly-trap collector. The free end tip portion of the conductive probe 20 preferably has as small a cross-sectional area as possible commensuratc with the voltage that must be held off between the anode 4 and the collector 5 and the power level.

In a preferred embodiment, the cross-sectional area of the tip of the probe 20 is preferably less than I percent of the cross-sectional area of the mouth 19 of the collector and, in a typical example, the tip area of the probe is 0.25 percent of the area of the mouth 19 when the maximum voltage between the accelerating anode 4 and the collector is 155 kV. The collector probe 20 has a half-cone angle of less than 20 and in a preferred embodiment the half-cone angle is 9.

The cone 20 is sealed to the closing endwall of the collector 5, as by a brazed joint therebetween, to form a gas-tight seal. The cone 20 is preferably hollow and contains an internal conical baffle 22 spaced from the inside wall of the cone 20 to define a fluid coolant passageway into the cone through the center of the conical baffle 22 and back out of the probe 20 via the annular space between the outside of the baffle 22 and the inside wall of the of the conical probe 20.

At the root portion of the probe 20 a plurality of apertures are formed in the probe to allow communication of the fluid coolant into a region between an external conical baffle 23 and the outside wall of the collector 5 such that the probe is placed in series with the coolant flow through the collector. Coolant flows between the end wall of the collector 5 and baffle 23 into a plenum chamber 24 and thence is exhausted from the chamber 24 via an exhaust port 25. Input coolant to the probe 20 and collector 5 is supplied to the inside of the conical baffle 22 via an input fluid coolant tubulation 26. The fluid coolant may comprise a gas or liquid such as oil, air, or water. In a typical example of a depressible collector design for switching l4 megawatts peak power and kilowatts average power, the collector coolant fluid was oil supplied at a flow rate of 3 gallons per minute.

Although, in a preferred embodiment, the collector probe 20 is shown as being conductively connected to the collector 5 for operation at the same potential, the collector probe 20, in an alternative embodiment, is connected to the collector cavity 5 via the intermediary of an electrical insulator member as of alumina ceramic (not shown) to hold off a potential applied between the collector probe 20 and the collector 5. A few kV potential applied to the probe 20 relative to the potential of the collector 5 may be utilized for adjusting the equipotentials within the collector 5 and at the mouth 19 of the collector 5 to aid in obtaining a uniform spread of the electron beam in the collector and to aid in control of the path of the secondary electrons to capture them within the fly-trap collector.

The beam focus electrode nose member 21 and the cooperating probe 20 are shaped and arranged at the mouth of the collector to produce a series of equipo tential surfaces across the beam path, as shown in the potential plot of FIG. 2,whieh preferably are generally mirror images of the equipotential surfaces in the beam path on the beam accelerating side of the accelerating electrode 4. In this manner, the electrons of the beam, at the beam edge, in the decelerating region, experience an inwardly directed force opposite to the outwardly directed force on such electrons produced by space charge within the beam. The ratio of these forces is chosen to provide a beam which expands as a desired function of axial distance, while preserving laminarity of the electron flow as far as possible.

The equipotential surfaces and the electron trajectories for the collector 5 of FIG. 1 are shown in the plot of FIG. 2 for a beam of I05 amps at a beam voltage of 155 kV with the collector 5 depressed by percent of the beam voltage, namely, to 7.75 kV. As is seen from the plot there is a space charge depression within the collector having a maximum area of depression near the mouth of the collector. The probe 20 and beam collector nose portion 21 are formed and arranged such that the equipotential surface, corresponding to the potential of the collector, namely 7.75 kV, bends across the mouth 19 of the collector in a spherically concave shape generally conforming to a mirror image of the concave shape of a similar equipotential located on the opposite side of the accelerating anode just overlaying the the cathode emitting surface 7.

The probe 20 and collector focus nose portion 21 are dimensioned and arranged to maintain laminar electron flow in the decelerating region of the electron beam as the beam expands into the mouth of the collector l9. ln addition, the electrodes 21 and 20 are arranged such that the expanding electron beam, at the mouth 19 of the collector 5, maintains a generally uniform current density taken transversely of the beam. The uniformity of the spacing of the individual electron trajectories in the plot of FIG. 2 shows that laminar flow and uniform current density in the beam are maintained as the beam expands into the collector.

The cone angle of the collector probe 20 is chosen such that a minimum number of electrons are collected on the probe and such that the space charged depression region is shaped to maintain the desired equipotential surfaces at the collector mouth 19. The shallow angle of incidence of primary electrons collected on the probe 20 and their points of collection well within the fly-trap collector further assure that secondary electrons liberated by collision of incident primary electrons are scattered off the probe 20 with trajectories which will take the secondary electrons back into the collector cavity, rather than backstreaming along the beam path to the anode 4 where they would be'collected at relatively high anode potential, thereby drastically reducing the efficiency of the tube.

In an alternative embodiment, not shown, the probe 20 is coated with a secondary electron emissive inhibiting material or the surface of the probe is grooved in such a manner that the secondary electrons liberated within the groove are captured on adjacent land portions of the surface rather than being liberated into the collector.

The linear beam tetrode switch tube 1 of FIG. 1 with electrodes shaped as aforedescribed has provided 95 percent efficiency when operated as a switch tube with a monoenergetic beam 3 under conditions of 98 percent collection of the beam current at 97 percent collector depression and with peak current levels to 100 amperes at depression voltages (anode to collector) up to 140 kV.

The switch tube of FIG. 1 is especially useful as a modulator tube for modulating the current to a thermionic diode load, such as the gun of a klystron, traveling wave tube, or other linear beam tube schematically indicated by diode 28 of FIG. 1, which is shown connected between the collector 5 and the accelerating anode 4. The accelerating anode 4 is operated at ground potential, which is also the positive potential of a beam power supply 29. This type of modulator circuit is disclosed and claimed in my copending US. applica tion Ser. No. 283,433 filed 24 Aug. 1972 and assigned to the same assignee as the present invention.

As used herein, high power is defined to mean power in excess of a 100 kW peak or 100 watts average.

While the above description contains many specificities, these should not be construed as limitations upon the scope of the invention, but merely as an exemplication of the preferred embodiments thereof. The intended scope of the invention is indicated by the following claims and their legal equivalents.

What is claimed is:

1. In a high power electron beam tube having a depressed potential collector:

thermionic cathode emitter means having a concave emitting surface for thermionically emitting electrons; centrally apertured accelerating anode means spaced from said emitter means and shaped for drawing a convergent stream of electrons through the central aperture of said anode in a substantially nonintercepting manner to form a laminar flow electron beam of generally uniform current density taken transversely across the beam path; collector means disposed at the terminal end of the beam path for collecting the electron beam, said collector operating at a depressed potential in respect to said anode means and having an electrically conductive cavity with a constricted mouth for receiving said beam in said cavity and for inhibiting egress of secondary and reflected electrons from said cavity, collector focusing means disposed at the mouth of said collector, and tapered conductive probe means disposed axially within said collector cavity, said probe having a terminating free end portion disposed approximately at the mouth of said collector cavity, said free end portion having a half-cone angle of less than 20 and a free end tip portion which has a transverse cross-sectional area for interception of beam current, said tip portion occupying less than 1 percent of the area of said constricted mouth of said collector cavity, said collector focusing means and said probe means being formed and arranged for shaping the equally potentials of the electron decelerating field at the mouth of said collector to maintain a generally laminar electron flow of uniform cross density taken transversely across the decelerated beam at the mouth of said collector cavity in the presence of regions of space charge depression within said collector cavity, whereby electrons will not be reflected out of said cavity and hence the efficiency of said tube will be substantially increased. 2. The apparatus of claim I wherein said cathode emitter has a generally spherically concave emitting surface, and wherein the decelerating electric field equipotential essentially at the mouth of said collector means is shaped by said collector focusing means and said probe means in the presence of space charge within said collector cavity to be approximately a mirror image of the corresponding equipotential overlaying the emitting surface of said cathode emitter.

3. The apparatus of claim 1 wherein said half-cone angle is approximately 9.

4. The apparatus of claim 1 wherein said tapered probe means is hollow so as to accommodate a flow of fluid coolant therethrough. v

5. The apparatus of claim 1 wherein the free end tip portion of said collector probe means terminates at the mouth of said collector just inside the collector cavity from the transverse plane of said mouth, and including means for operating said probe at the same depressed potential as said beam collector cavity.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION e PATENT NO. I 3 7 0 33 DATED December 18, 19 73 INV ENTOR(S) ROBERT HENRY GIEBLER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, lines 31-32 change "equally potentials" to equipotentials Column 6, line 34 change "cross" to current Signed and Sealed this Sixth Day of July 1976 9 [SEAL] Arrest.

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Parents and Trademarks 

1. In a high power electron beam tube having a depressed potential collector: thermionic cathode emitter means having a concave emitting surface for thermionically emitting electrons; centrally apertured accelerating anode means spaced from said emitter means and shaped for drawing a convergent stream of electrons through the central aperture of said anode in a substantially non-intercepting manner to form a laminar flow electron beam of generally uniform current density taken transversely across the beam path; collector means disposed at the terminal end of the beam path for collecting the electron beam, said collector operating at a depressed potential in respect to said anode means and having an electrically conductive cavity with a constricted mouth for receiving said beam in said cavity and for inhibiting egress of secondary and reflected electrons from said cavity, collector focusing means disposed at the mouth of said collector, and tapered conductive probe means disposed axially within said collector cavity, said probe having a terminating free end portion disposed approximately at the mouth of said collector cavity, said free end portion having a half-cone angle of less than 20* and a free end tip portion which has a transverse cross-sectional area for interception of beam current, said tip portion occupying less than 1 percent of the area of said constricted mouth of said collector cavity, said collector focusing means and said probe means being formed and arranged for shaping the equally potentials of the electron decelerating field at the mouth of said collector to maintain a generally laminar electron flow of uniform cross density taken transversely across the decelerated beam at the mouth of said collector cavity in the presence of regions of space charge depression within said collector cavity, whereby electrons will not be reflected out of said cavity and hence the efficiency of said tube will be substantially increased.
 2. The apparatus of claim 1 wherein said caThode emitter has a generally spherically concave emitting surface, and wherein the decelerating electric field equipotential essentially at the mouth of said collector means is shaped by said collector focusing means and said probe means in the presence of space charge within said collector cavity to be approximately a mirror image of the corresponding equipotential overlaying the emitting surface of said cathode emitter.
 3. The apparatus of claim 1 wherein said half-cone angle is approximately 9*.
 4. The apparatus of claim 1 wherein said tapered probe means is hollow so as to accommodate a flow of fluid coolant therethrough.
 5. The apparatus of claim 1 wherein the free end tip portion of said collector probe means terminates at the mouth of said collector just inside the collector cavity from the transverse plane of said mouth, and including means for operating said probe at the same depressed potential as said beam collector cavity. 